The operation of water turbines is generally controlled by means of a mobile guide vane assembly which fulfills three functions:
The deflection of the water during normal operation;
The control of the flow variations during starting up and stopping;
The provision of water tight sealing in the completely closed position when the machine is stopped.
Such guide vane assemblies (and in particular the conical guide vane assemblies of bulb type turbines) are difficult to manufacture if they are to fulfill these three functions.